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SQL As Understood By SQLite

(This page was last modified on 2001/12/22 19:27:41 UTC)

The SQLite library understands most of the standard SQL language. But it does omit some features while at the same time adding a few features of its own. This document attempts to describe percisely what parts of the SQL language SQLite does and does not support.

In all of the syntax diagrams that follow, literal text is shown in bold blue. Non-terminal symbols are shown in italic red. Operators that are part of the syntactic markup itself are shown in black roman.

This document is just an overview of the SQL syntax implemented by SQLite. Many low-level productions are omitted. For detailed information on the language that SQLite understands, refer to the source code.

SQLite implements the follow SQL commands:

 

 

Details on the implementation of each command are provided in the sequel.


BEGIN TRANSACTION

sql-statement ::= BEGIN [TRANSACTION [name]]
sql-statement ::= END [TRANSACTION [name]]
sql-statement ::= COMMIT [TRANSACTION [name]]
sql-statement ::= ROLLBACK [TRANSACTION [name]]

Beginning in version 2.0, SQLite supports transactions with rollback and atomic commit. However, only a single level of transaction is allowed. Transactions may not be nested.

No changes can be made to the database except within a transaction. Any command that changes the database (basically, any SQL command other than SELECT) will automatically starts a transaction if one is not already in effect. Automatically stared transactions are committed at the conclusion of the command.

Transactions can be started manually using the BEGIN TRANSACTION command. Such transactions persist until a COMMIT or ROLLBACK or until an error occurs or the database is closed. If an error is encountered or the database is closed, the transaction is automatically rolled back. The END TRANSACTION command is a alias for COMMIT.


COPY

sql-statement ::= COPY table-name FROM filename

The COPY command is an extension used to load large amounts of data into a table. It is modeled after a similar command found in PostgreSQL. In fact, the SQLite COPY command is specifically designed to be able to read the output of the PostgreSQL dump utility pg_dump so that data can be easily transferred from PostgreSQL into SQLite.

 

The table-name is the name of an existing table which is to be filled with data. The filename is a string or identifier that names a file from which data will be read. The filename can be the STDIN to read data from standard input.

 

Each line of the input file is converted into a single record in the table. Columns are separated by tabs. If a tab occurs as data within a column, then that tab is preceded by a baskslash "\" character. A baskslash in the data appears as two backslashes in a row.

When the input data source is STDIN, the input can be terminated by a line that contains only a baskslash and a dot: "\.".


CREATE INDEX

sql-statement ::= CREATE [UNIQUE] INDEX index-name
ON
table-name ( column-name [, column-name]* )
column-name ::= name [ ASC | DESC ]

The CREATE INDEX command consists of the keywords "CREATE INDEX" followed by the name of the new index, the keyword "ON", the name of a previously created table that is to be indexed, and a parenthesized list of names of columns in the table that are used for the index key. Each column name can be followed by one of the "ASC" or "DESC" keywords to indicate sort order, but the sort order is ignored in the current implementation.

There are no arbitrary limits on the number of indices that can be attached to a single table, nor on the number of columns in an index.

If the UNIQUE keyword appears between CREATE and INDEX then duplicate index entries are not allowed. Any attempt to insert a duplicate entry will result in a rollback and an error message.

The exact text of each CREATE INDEX statement is stored in the sqlite_master table. Everytime the database is opened, all CREATE INDEX statements are read from the sqlite_master table and used to regenerate SQLite's internal representation of the index layout.


CREATE TABLE

sql-command ::= CREATE [TEMP | TEMPORARY] TABLE table-name (
    
column-def [, column-def]*
    
[, constraint]*
)
column-def ::= name type [column-constraint]*
type ::= typename |
typename ( number ) |
typename ( number , number )
column-constraint ::= NOT NULL |
PRIMARY KEY
[sort-order] |
UNIQUE
|
CHECK (
expr ) |
DEFAULT
value
constraint ::= PRIMARY KEY ( name [, name]* ) |
UNIQUE (
name [, name]* ) |
CHECK (
expr )

A CREATE TABLE statement is basically the keywords "CREATE TABLE" followed by the name of a new table and a parenthesized list of column definitions and constraints. The table name can be either an identifier or a string. The only reserved table name is "sqlite_master" which is the name of the table that records the database schema.

Each column definition is the name of the column followed by the datatype for that column, then one or more optional column constraints. The datatype for the column is ignored. All information is stored as null-terminated strings. The UNIQUE constraint causes an index to be created on the specified columns. This index must contain unique keys. The DEFAULT constraint specifies a default value to use when doing an INSERT.

Specifying a PRIMARY KEY normally just creates a UNIQUE index on the primary key. However, if primary key is on a single column that has datatype INTEGER, then that column is used internally as the actual key of the B-Tree for the table. This means that the column may only hold unique integer values. (Except for this one case, SQLite ignores the datatype specification of columns and allows any kind of data to be put in a column regardless of its declared datatype.) If a table does not have an INTEGER PRIMARY KEY column, then the B-Tree key will be a randomly generated integer. The B-Tree key for a row can always be accessed using one of the special names "ROWID", "OID", or "_ROWID_". This is true regardless of whether or not there is an INTEGER PRIMARY KEY.

If the "TEMP" or "TEMPORARY" keyword occurs in between "CREATE" and "TABLE" then the table that is created is only visible to the process that opened the database and is automatically deleted when the database is closed. Any indices created on a temporary table are also temporary. Temporary tables and indices are stored in a separate file distinct from the main database file.

There are no arbitrary limits on the number of columns or on the number of constraints in a table. The total amount of data in a single row is limited to about 1 megabytes. (This limit can be increased to 16MB by changing a single #define in the source code and recompiling.)

The exact text of each CREATE TABLE statement is stored in the sqlite_master table. Everytime the database is opened, all CREATE TABLE statements are read from the sqlite_master table and used to regenerate SQLite's internal representation of the table layout.


DELETE

sql-statement ::= DELETE FROM table-name [WHERE expression]

The DELETE command is used to remove records from a table. The command consists of the "DELETE FROM" keywords followed by the name of the table from which records are to be removed.

Without a WHERE clause, all rows of the table are removed. If a WHERE clause is supplied, then only those rows that match the expression are removed.


DROP INDEX

sql-command ::= DROP INDEX index-name

The DROP INDEX statement consists of the keywords "DROP INDEX" followed by the name of the index. The index named is completely removed from the disk. The only way to recover the index is to reenter the appropriate CREATE INDEX command.


DROP TABLE

sql-command ::= DROP TABLE table-name

The DROP TABLE statement consists of the keywords "DROP TABLE" followed by the name of the table. The table named is completely removed from the disk. The table can not be recovered. All indices associated with the table are also deleted.


EXPLAIN

sql-statement ::= EXPLAIN sql-statement

The EXPLAIN command modifier is a non-standard extension. The idea comes from a similar command found in PostgreSQL, but the operation is completely different.

If the EXPLAIN keyword appears before any other SQLite SQL command then instead of actually executing the command, the SQLite library will report back the sequence of virtual machine instructions it would have used to execute the command had the EXPLAIN keyword not been present. For additional information about virtual machine instructions see the architecture description or the documentation on available opcodes for the virtual machine.


expression

expression ::= expression binary-op expression |
expression like-op expression |
unary-op expression |
(
expression ) |
column-name |
table-name . column-name |
literal-value |
function-name ( expr-list | * ) |
expression ISNULL |
expression NOTNULL |
expression [NOT] BETWEEN expression AND expression |
expression [NOT] IN ( value-list ) |
expression [NOT] IN ( select ) |
(
select )
like-op ::= LIKE | GLOB | NOT LIKE | NOT GLOB

This section is different from the others. Every other section of this document talks about a particular SQL command. This section does not talk about a standalone command but about "expressions" which are subcomponent of most other commands.

SQLite understands the following binary operators, in order from highest to lowest precedence:

*    /    %
+    -
<<   >>   &    |
<    <=   >    >=
=    ==   !=   <>   IN
AND
OR

Supported unary operaters are these:

-    +    !    ~

Any SQLite value can be used as part of an expression. For arithmetic operations, integers are treated as integers. Strings are first converted to real numbers using atof(). For comparison operators, numbers compare as numbers and strings compare as strings. For string comparisons, case is significant but is only used to break a tie. Note that there are two variations of the equals and not equals operators. Equals can be either = or ==. The non-equals operator can be either != or <>.

The LIKE operator does a wildcard comparision. The operand to the right contains the wildcards. A percent symbol % in the right operand matches any sequence of zero or more characters on the left. An underscore _ on the right matches any single character on the left. The LIKE operator is not case sensitive and will match upper case characters on one side against lower case characters on the other. (A bug: SQLite only understands upper/lower case for 7-bit Latin characters. Hence the LIKE operator is case sensitive for 8-bit iso8859 characters or UTF-8 characters. For example, the expression 'a' LIKE 'A' is TRUE but 'æ' LIKE 'Æ' is FALSE.)

The GLOB operator is similar to LIKE but uses the Unix file globbing syntax for its wildcards. Also, GLOB is case sensitive, unlike LIKE. Both GLOB and LIKE may be preceded by the NOT keyword to invert the sense of the test.

A column name can be any of the names defined in the CREATE TABLE statement or one of the following special identifiers: "ROWID", "OID", or "_ROWID_". These special identifiers all describe the unique random integer key (the "row key") associated with every row of every table. The special identifiers only refer to the row key if the CREATE TABLE statement does not define a real column with the same name. Row keys act like read-only columns. A row key can be used anywhere a regular column can be used, except that you cannot change the value of a row key in an UPDATE or INSERT statement. "SELECT * ..." does not return the row key.

SELECT statements can appear in expressions as either the right-hand operand of the IN operator or as a scalar quantity. In both cases, the SELECT should have only a single column in its result. Compound SELECTs (connected with keywords like UNION or EXCEPT) are allowed. Any ORDER BY clause on the select is ignored. A SELECT in an expression is evaluated once before any other processing is performed, so none of the expressions within the select itself can refer to quantities in the containing expression.

When a SELECT is the right operand of the IN operator, the IN operator returns TRUE if the result of the left operand is any of the values generated by the select. The IN operator may be preceded by the NOT keyword to invert the sense of the test.

When a SELECT appears within an expression but is not the right operand of an IN operator, then the first row of the result of the SELECT becomes the value used in the expression. If the SELECT yields more than one result row, all rows after the first are ignored. If the SELECT yeilds no rows, then the value of the SELECT is NULL.

The expression syntax currently supports the following functions:

count    min       max       sum
avg      length    substr    abs       round

The functions count, sum, and avg and the functions min and max used with only one argument are all aggregate functions. This means that they are computed across all rows of the result. The functions min and max with two or more arguments and all other functions are non-aggregates. Non-aggregate functions are computed separately for each row of the result.

The round function can take either 1 or 2 arguments. The first argument is the floating point value that is rounded. The second argument is the number of digits to the right of the decimal point to preserve. If the second argument is omitted, zero is assumed. So round(1.23456,2) is 1.23 and round(12.34,0) and round(12.34) both evaluate to 12.

The "count(*)" syntax is supported but "count(distinct COLUMN-NAME)" is not.


INSERT

sql-statement ::= INSERT INTO table-name [( column-list )] VALUES ( value-list ) |
INSERT INTO
table-name [( column-list )] select-statement

The INSERT statement comes in two basic forms. The first form (with the "VALUES" keyword) creates a single new row in an existing table. If no column-list is specified then the number of values must be the same as the number of columns in the table. If a column-list is specified, then the number of values must match the number of specified columns. Columns of the table that do not appear in the column list are fill with the default value, or with NULL if not default value is specified.

The second form of the INSERT statement takes it data from a SELECT statement. The number of columns in the result of the SELECT must exactly match the number of columns in the table if no column list is specified, or it must match the number of columns name in the column list. A new entry is made in the table for every row of the SELECT result. The SELECT may be simple or compound. If the SELECT statement has an ORDER BY clause, the ORDER BY is ignored.


SELECT

sql-statement ::= SELECT result FROM table-list
[WHERE expression]
[GROUP BY expr-list]
[HAVING expression]
[compound-op select]*
[ORDER BY sort-expr-list]
[LIMIT integer [OFFSET integer]]
result ::= * | result-column [, result-column]*
result-column ::= expression [ [AS] string ]
table-list ::= table-name [, table-name]*
sort-expr-list ::= expr [sort-order] [, expr [sort-order]]*
sort-order ::= ASC | DESC
compound_op ::= UNION | UNION ALL | INTERSECT | EXCEPT

The SELECT statement is used to query the database. The result of a SELECT is zero or more rows of data where each row has a fixed number of columns. The number of columns in the result is specified by the expression list in between the SELECT and FROM keywords. Any arbitrary expression can be used as a result. If the result specification is just * then all columns of all tables are used as the result.

The query is executed again one or more tables specified after the FROM keyword. If more than one table is specified, then the query is against the join of the various tables.

The WHERE clause can be used to limit the number of rows over which the query operates. In the current implementation, indices will only be used to optimize the query if WHERE expression contains equality comparisons connected by the AND operator.

The GROUP BY clauses causes one or more rows of the result to be combined into a single row of output. This is especially useful when the result contains aggregate functions. The expressions in the GROUP BY clause do not have to be expressions that appear in the result. The HAVING clause is similar to WHERE except that HAVING applies after grouping has occurred. The HAVING expression may refer to values, even aggregate functions, that are not in the result.

The ORDER BY clause causes the output rows to be sorted. The argument to ORDER BY is a list of expressions that are used as the key for the sort. The expressions do not have to be part of the result for a simple SELECT, but in a compound SELECT each sort expression must exactly match one of the result columns. Each sort expression may be optionally followed by ASC or DESC to specify the sort order.

The LIMIT clause places an upper bound on the number of rows returned in the result. A LIMIT of 0 indicates no upper bound. The optional OFFSET following LIMIT specifies how many rows to skip at the beginning of the result set.

A compound SELECT is formed from two or more simple SELECTs connected by one of the operators UNION, UNION ALL, INTERSECT, or EXCEPT. In a compound SELECT, all the constituent SELECTs must specify the same number of result columns. There may be only a single ORDER BY clause at the end of the compound SELECT. The UNION and UNION ALL operators combine the results of the SELECTs to the right and left into a single big table. The difference is that in UNION all result rows are distinct where in UNION ALL there may be duplicates. The INTERSECT operator takes the intersection of the results of the left and right SELECTs. EXCEPT takes the result of left SELECT after removing the results of the right SELECT. When three are more SELECTs are connected into a compound, they group from left to right.


UPDATE

sql-statement ::= UPDATE table-name SET assignment [, assignment] [WHERE expression]
assignment ::= column-name = expression

The UPDATE statement is used to change the value of columns in selected rows of a table. Each assignment in an UPDATE specifies a column name to the left of the equals sign and an arbitrary expression to the right. The expressions may use the values of other columns. All expressions are evaluated before any assignments are made. A WHERE clause can be used to restrict which rows are updated.


VACUUM

sql-statement ::= VACUUM [index-or-table-name]

The VACUUM command is an SQLite extension modelled after a similar command found in PostgreSQL. If VACUUM is invoked with the name of a table or index then it is suppose to clean up the named table or index. In version 1.0 of SQLite, the VACUUM command would invoke gdbm_reorganize() to clean up the backend database file. Beginning with version 2.0 of SQLite, GDBM is no longer used for the database backend and VACUUM has become a no-op.


PRAGMA

sql-statement ::= PRAGMA name = value |
PRAGMA
function(arg)

The PRAGMA command is used to modify the operation of the SQLite library. The pragma command is experimental and specific pragma statements may removed or added in future releases of SQLite. Use this command with caution.

The current implementation supports the following pragmas:

  • PRAGMA cache_size = Number-of-pages;

    Change the maximum number of database disk pages that SQLite will hold in memory at once. Each page uses about 1.5K of RAM. The default cache size is 100. If you are doing UPDATEs or DELETEs that change many rows of a database and you do not mind if SQLite uses more memory, you can increase the cache size for a possible speed improvement.

  • PRAGMA count_changes = ON;
    PRAGMA count_changes = OFF;

    When on, the COUNT_CHANGES pragma causes the callback function to be invoked once for each DELETE, INSERT, or UPDATE operation. The argument is the number of rows that were changed.

  • PRAGMA empty_result_callbacks = ON;
    PRAGMA empty_result_callbacks = OFF;

    When on, the EMPTY_RESULT_CALLBACKS pragma causes the callback function to be invoked once for each query that has an empty result set. The third "argv" parameter to the callback is set to NULL because there is no data to report. But the second "argc" and fourth "columnNames" parameters are valid and can be used to determine the number and names of the columns that would have been in the result set had the set not been empty.

  • PRAGMA full_column_names = ON;
    PRAGMA full_column_names = OFF;

    The column names reported in an SQLite callback are normally just the name of the column itself, except for joins when "TABLE.COLUMN" is used. But when full_column_names is turned on, column names are always reported as "TABLE.COLUMN" even for simple queries.

  • PRAGMA index_info(index-name);

    For each column that the named index references, invoke the callback function once with information about that column, including the column name, and the column number.

  • PRAGMA index_list(table-name);

    For each index on the named table, invoke the callback function once with information about that index. Arguments include the index name and a flag to indicate whether or not the index must be unique.

  • PRAGMA parser_trace = ON;
    PRAGMA parser_trace = OFF;

    Turn tracing of the SQL parser inside of the SQLite library on and off. This is used for debugging. This only works if the library is compiled without the NDEBUG macro.

  • PRAGMA table_info(table-name);

    For each column in the named table, invoke the callback function once with information about that column, including the column name, data type, whether or not the column can be NULL, and the default value for the column.

  • PRAGMA vdbe_trace = ON;
    PRAGMA vdbe_trace = OFF;

    Turn tracing of the virtual database engine inside of the SQLite library on and off. This is used for debugging.

No error message is generated if an unknown pragma is issued. Unknown pragmas are ignored.